1. Field of the Invention
In general the invention relates to the field of inkjet printing. More particularly, the invention relates to a method and apparatus for controlling lightness in colored inkjet inks by means of a transparent white ink composition.
2. Background Information
Conventionally, in color inkjet printing, the printer must print several of the printer's dots for each pixel of an image. Inkjet printers may print only a few colors: for example, cyan, magenta, yellow and black. Rendering each pixel of an image into various colors, shades, and intensities requires that the image be dithered, meaning that the printer creates patterns of dots in multiple colors in order to simulate the color of each pixel in the image. Additionally, some inkjet printers also print lighter values of the same four colors. Thus, conventional inkjet printers now typically use eight colors: cyan, light cyan, magenta, light magenta yellow, light yellow, black and light black. Currently, an inkjet printer's ability to output these eight different colors requires an inkjet set that includes at least eight separate inks, with a separate channel devoted to each ink. Thus, modern inkjet printers must have at least eight print channels. The cost to own and operate a conventional inkjet printer reflects the manufacturing cost of providing a separate channel for each of the colors.
Additionally, large-format or grand-format UV inkjet printers that are capable of printing with UV-curable white ink are becoming increasingly common. The white-ink printing capability, requiring at least one additional print channel, adds to the design complexity and the manufacturing cost of such inkjet printers.
Inkjet printing methods wherein inkjet inks are mixed with colorless liquids just prior to jetting in order to control color density are known. Additionally, it is known to produce a white inkjet ink having improved dispersion stability by mixing a white inkjet ink having a predetermined refractive index with at least one colorless liquid, such as a dispersant, a photo-initiator or a solvent.